Excerpt from CS Monitor, "Why no safe zone in Syria, yet?""As I explored in a piece for the United States Naval Institute, imposing a no-fly zone in Syria would likely mean conducting intensive Suppression of Enemy Air Defense to destroy Syria’s air defenses and air force. Even a partial no-fly zone would likely require some strikes outside its limits in order to degrade Syrian airfields, early-warning radars and mobile or semi-mobile air defense systems....Additionally, any proposal for military action needs to be understood in a fully comprehensive manner. A no-fly zone (NFZ) cannot be established without some air strikes to eliminate air defenses. Any air strikes would require an NFZ that would allow freedom of action by the forces involved." (Emphasis Added)

So in a "No Fly Zone," NATO would fly around destroying Syrian air defenses.

For the more suspicious folks out there, NATO would shoot anything that "appeared" threatening as well.

...It could act as "cover" to assasinate Assad...which is why Russia would veto a "No Fly Zone."